The instant invention relates to generally to useful oxygen electrode active materials for fuel cells, more specifically to their use as the oxygen electrode material for Ovonic instant startup alkaline fuel cells. These inventive oxygen electrodes open up a tremendous number of degrees of freedom in fuel cell design by utilizing reduction/oxidation (redox) couples, such as metal/oxide couples, or simply couples which provide electrochemical oxidizer, preferably oxygen, to the fuel cell electrolyte for electrochemical xe2x80x9ccombustionxe2x80x9d. These redox couples, due to their electrochemical potential, provide the fuel cells employing them with an increased operating voltage that is adjustable by varying the redox couple used. Additionally the redox couple provides the fuel cell with the ability to store oxygen within the electrode which not only provides for instant startup, but also provides the capability to provide short surge bursts of energy during demand surges and also allows for recapture of regenerative energy. The present enhanced fuel cell oxygen electrode materials display an increased resistance to dissolution in the fuel cell electrolyte and an increased catalytic activity toward the dissociation of molecular oxygen into atomic oxygen.
As the world""s human population expands, greater amounts of energy are necessary to provide the economic growth all nations desire. The traditional sources of energy are the fossil fuels which, when consumed, create significant amounts of carbon dioxide as well as other more immediately toxic materials including carbon monoxide, sulfur oxides, and nitrogen oxides. Increasing atmospheric concentrations of carbon dioxide are warming the earth; creating the ugly specter of global climatic changes. Energy-producing devices which do not contribute to such difficulties are needed now.
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Highly efficient fuel cells employing hydrogen, particularly with their simple combustion product of water, would seem an ideal alternative to current typical power generations means. Researchers have been actively studying such devices to utilize the fuel cell""s potential high energy-generation efficiency.
The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Presently most of the fuel cell R and D is focused on P.E.M. (Proton Exchange Membrane) fuel cells. Regrettably, the P.E.M. fuel cell suffers from relatively low conversion efficiency and has many other disadvantages. For instance, the electrolyte for the system is acidic. Thus, noble metal catalysts are the only useful active materials for the electrodes of the system. Unfortunately, not only are the noble metals costly, they are also susceptible to poisoning by many gases, specifically carbon monoxide (CO). Also, because of the acidic nature of the P.E.M fuel cell electrolyte, the remainder of the materials of construction of the fuel cell need to be compatible with such an environment, which again adds to the cost and choice of materials thereof. The proton exchange membrane itself is quite expensive, and because of it""s low proton conductivity at temperatures below 80xc2x0 C., inherently limits the power performance and operational temperature range of the P.E.M. fuel cell as the PEM is nearly non-functional at low temperatures. Also, the membrane is sensitive to high temperatures, and begins to soften at 120xc2x0 C. The membrane""s conductivity depends on water and has a tendency to dry out at higher temperatures, thus causing cell failure. Therefore, there are many disadvantages to the P.E.M. fuel cell which make it somewhat undesirable for commercial/consumer use.
The conventional alkaline fuel cells have some advantages over P.E.M. fuel cells in that they have higher operating efficiencies, they use less costly materials of construction, and they have no need for expensive membranes. The alkaline fuel cell also enjoys the relatively higher ionic conductivity of the electrolyte, and therefore has a much higher power capability. The kinetics within the alkaline are also much faster as compared to a P.E.M. fuel cell. While the conventional alkaline fuel cell is less sensitive to temperature than the PEM fuel cell, the platinum active materials of conventional alkaline fuel cell electrodes become very inefficient at low temperatures. Unfortunately, conventional alkaline fuel cells still suffer from their own disadvantages.
For example, conventional alkaline fuel cells still use expensive noble metal catalysts in both electrodes, which, as in the P.E.M. fuel cell, are susceptible to gaseous contaminant poisoning. The conventional alkaline fuel cell is also susceptible to the formation of carbonates from CO2 produced by oxidation of the anode carbon substrates or introduced via impurities in the fuel and air used at the electrodes. This carbonate formation clogs the electrolyte/electrode surface and reduces/eliminates the activity thereof. In addition it can also reduce the conductivity of the electrolyte. The invention described herein eliminates this problem from the anode.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous anode and oxygen electrode and brought into surface contact with the electrolytic solution. The particular materials utilized for the oxygen electrode and anode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the anode occurs between the hydrogen fuel and hydroxyl ions (OHxe2x88x92) present in the electrolyte, which react to form water and release electrons:
H2+2OHxe2x88x92xe2x86x922H2O+2exe2x88x92 E0=xe2x88x920.828 v. 
At the oxygen electrode, the oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OHxe2x88x92):
O2+2H2O+4exe2x88x92xe2x86x924OHxe2x88x92 E0=xe2x88x920.401 v. 
The total reaction, therefore, is:
2H2=O2xe2x86x922H2O E0=xe2x88x921.229 v 
The flow of electrons is utilized to provide electrical energy for a load externally connected to the anode and oxygen electrode.
It should be noted that the anode catalyst of the alkaline fuel cell is required to do more than catalyze the reaction of H+ ions with OHxe2x88x92 ions to produce water. Initially the anode must catalyze and accelerate the formation of H+ ions and release exe2x88x92 from H2. This occurs via formation of atomic hydrogen from molecular hydrogen. The overall reaction may be simplified and presented (where M is the catalyst) as:
M+H2xe2x86x922M . . . Hxe2x86x92M+2H++2exe2x88x92. 
Thus the anode catalyst must not only efficiently catalyze the electrochemical reaction for the formation of water at the electrolyte interface but must also efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional anode material, the dissociated hydrogen is transitional and the hydrogen atoms can easily recombine to form molecular hydrogen if they are not used very efficiently in the oxidation reaction. With the hydrogen storage anode materials of the inventive instant startup fuel cells, hydrogen is stored in hydride form as soon as they are created, and then are used as needed to provide power.
In addition to being catalytically efficient on both interfaces, the catalytic material must be resistant to corrosion by the alkaline electrolyte. Without such corrosion resistance, the electrode would quickly succumb to the harsh environment and the cell would quickly lose efficiency and die.
One prior art fuel cell anode catalyst is platinum. Platinum, despite its good catalytic properties, is not very suitable for wide scale commercial use as a catalyst for fuel cell anodes, because of its very high cost, and the limited world supply. Also, noble metal catalysts like platinum, also cannot withstand contamination by impurities normally contained in the hydrogen fuel stream. These impurities can include carbon monoxide which may be present in hydrogen fuel or contaminants contained in the electrolyte such as the impurities normally contained in untreated water including calcium, magnesium, iron, and copper.
The above contaminants can cause what is commonly referred to as a xe2x80x9cpoisoningxe2x80x9d effect. Poisoning occurs where the catalytically active sites of the material become inactivated by poisonous species that effectively block the active sites from being active. Once the catalytically active sites are inactivated, they are no longer available for acting as catalysts for efficient hydrogen oxidation reaction at the anode. The catalytic efficiency of the anode therefore is reduced since the overall number of available catalytically active sites is significantly lowered by poisoning. In addition, the decrease in catalytic activity results in increased over-voltage at the anode and hence the cell is much less efficient adding significantly to the operating costs. Overvoltage is the difference between the actual working electrode potential under some conditions and it""s equilibrium value, the physical meaning of overvoltage is the voltage required to overcome the resistance to the passage of current at the surface of the anode (charge transfer resistance). The overvoltage represents an undesirable energy loss which lowers the operating efficiency and adds to the operating costs of the fuel cell.
In related work, U.S. Pat. No. 4,623,597 (xe2x80x9cthe ""597 patentxe2x80x9d) and others in it""s lineage, the disclosure of which is hereby incorporated by reference, one of the present inventors, Stanford R. Ovshinsky, described disordered multi-component multi-phase hydrogen storage materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor made (i.e., atomically engineered) to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are amorphous, microcrystalline, intermediate range order, and/or polycrystalline (lacking long range compositional order) wherein the polycrystalline material includes topological, compositional, translational, and positional modification and disorder. The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites.
The disordered electrode materials of the ""597 patent were formed from lightweight, low cost elements by any number of techniques, which assured formation of primarily non-equilibrium metastable phases resulting in the high energy and power densities and low cost. The resulting low cost, high energy density disordered material allowed the batteries to be utilized most advantageously as secondary batteries, but also as primary batteries.
Tailoring of the local structural and chemical order of the materials of the ""597 patent was of great importance to achieve the desired characteristics. The improved characteristics of the anodes of the ""597 patent were accomplished by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix to create a desired disordered material. Disorder permits degrees of freedom, both of type and of number, within a material, which are unavailable in conventional materials. These degrees of freedom dramatically change a materials physical, structural, chemical and electronic environment. The disordered material of the ""597 patent have desired electronic configurations which result in a large number of active sites. The nature and number of storage sites were designed independently from the catalytically active sites.
Multiorbital modifiers, for example transition elements, provide a greatly increased number of storage sites due to various bonding configurations available, thus resulting in an increase in energy density. The technique of modification especially provides non-equilibrium materials having varying degrees of disorder provided unique bonding configurations, orbital overlap and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural rearrangement occurs during charge/discharge cycles or rest periods there between resulting in long cycle and shelf life.
The improved battery of the ""597 patent included electrode materials having tailor-made local chemical environments which were designed to yield high electrochemical charging and discharging efficiency and high electrical charge output. The manipulation of the local chemical environment of the materials was made possible by utilization of a host matrix which could, in accordance with the ""597 patent, be chemically modified with other elements to create a greatly increased density of electro-catalytically active sites and hydrogen storage sites.
The disordered materials of the ""597 patent were designed to have unusual electronic configurations, which resulted from the varying 3-dimensional interactions of constituent atoms and their various orbitals. The disorder came from compositional, positional and translational relationships of atoms. Selected elements were utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
The internal topology that was generated by these configurations also allowed for selective diffusion of atoms and ions. The invention that was described in the ""597 patent made these materials ideal for the specified use since one could independently control the type and number of catalytically active and storage sites. All of the aforementioned properties made not only an important quantitative difference, but qualitatively changed the materials so that unique new materials ensued.
Disorder can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions of the material. The disorder also can be introduced by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, in addition to regions of a crystalline phase or phases. The interfaces between these various phases can provide surfaces which are rich in local chemical environments which provide numerous desirable sites for electrochemical hydrogen storage.
These same principles can be applied within a single structural phase. For example, compositional disorder is introduced into the material which can radically alter the material in a planned manner to achieve important improved and unique results, using the Ovshinsky principles of disorder on an atomic or microscopic scale.
Additionally, in copending U.S. application Ser. No. 09/524,116, (""116), the disclosure of which is hereby incorporated by reference, Ovshinsky has employed the principles of atomic engineering to tailor materials which uniquely and dramatically advance the fuel cell art. The invention of ""116 application has met a need for materials which allow fuel cells to startup instantaneously by providing an internal source of fuel, to operate in a wide range of ambient temperatures to which a fuel cell will be exposed to under ordinary consumer use and to allow the fuel cell to be run in reverse as an electrolyzer thereby utilizing/storing recaptured energy. The anodes of the ""116 fuel cells are formed from relatively inexpensive hydrogen storage materials which are highly catalytic to the dissociation of molecular hydrogen and the formation of water from hydrogen and hydroxyl ions as well as being corrosion resistant to the electrolyte, resistant to contaminant poisoning from the reactant stream and capable of working in a wide temperature range.
The next step in the evolution of the fuel cell was to find suitable materials to replace the expensive platinum oxygen electrode catalysts of conventional fuel cells. This occurred with the invention of copending U.S. patent application Ser. No. 09/797,332 filed Mar. 1, 2001 entitled xe2x80x9cNOVEL FUEL CELL CATHODES AND THEIR FUEL CELLSxe2x80x9d, the disclosure of which is hereby incorporated by reference. These oxygen electrodes have the ability to store chemical energy (in the form of chemically bound oxygen) to assist in the instant startup of the fuel cell as well as recapture energy The invention described the copending application was significant in that it provided the next step in the development of fuel cells. With that invention, the oxygen electrode could be selected from a broad menu of available possible redox couples. These redox couples in addition to providing a store of chemical energy, allow the operating voltage of the fuel cell to be selected, by judicious choice of the redox couple used.
While these improved oxygen electrodes are very useful, they have some problems that up to now have not been addressed. Namely, the redox couple material can sometimes be somewhat soluble in the fuel cell electrolyte solution. Additionally, the redox couple may need it""s catalytic activities matched. That is, the redox couple materials may have a faster rate of electrochemical activity than their gas phase activity, or vice versa. Thus there is a need in the art to increase the stability of the active redox couple material toward dissolution in the electrolyte and to match the gas phase kinetics of the active redox couple material with its electrochemical kinetics.
The present invention discloses a fuel cell which has the ability to start up instantly and can accept recaptured energy such as that of regenerative braking by operating in reverse as an electrolyzer. The instant startup fuel cells have increased efficiency and power availability (higher voltage and current) and a dramatic improvement in operating temperature range (xe2x88x9220 to 150xc2x0 C.) The fuel cells of the instant invention also have additional degrees of freedom over the fuel cells of the prior art in that the voltage output of the cell can be tailored and they are capable of storing regenerated energy.
The oxygen electrodes of the instant fuel cells operate through the mechanism of redox reactions which uniquely provide multiple degrees of freedom in selecting the operating voltages available for such fuel cells. Such oxygen electrodes provide the fuel cells in which they are used, particularly alkaline fuel cells, with a level of chemical energy storage within the oxygen electrode itself. This means that such fuel cells will have a xe2x80x9cbufferxe2x80x9d or xe2x80x9cchargexe2x80x9d available within the oxygen electrode at all times.
The fuel cell oxygen electrodes of the instant invention employ modified active redox couple materials which exhibit increased stability of the active redox couple material toward dissolution in the electrolyte and which match the gas phase kinetics of the active redox couple material with its electrochemical kinetics. Specific modifiers which may be added to the redox couples includes elements such as Li, B, Si, Al, Ni, Cr, Mn, Mo, Misch Metal, Sn, Hg, Pb, Ga, and alloys thereof.
The electrode has a first surface region situated to allow exposure to molecular oxygen which includes a catalytically acting component promoting the absorption of oxygen through said first surface region and into said active material to chemically charge said active material through oxygen absorption.
The fuel cell oxygen electrodes of this invention may utilize redox couples, particularly metal/oxides couples selected from the group of metals consisting of copper, silver, zinc, cobalt and cadmium. Another useful redox couple is the nickel hydroxide/nickel oxyhydroxide couple disclosed herein above.